Recently, magneto-optical recording media such as magneto-optical disks and magneto-optical cards have been considered as prospective rewritable large-capacity optical memory.
Information is recorded on a magneto-optical recording medium by applying an external magnetic field onto a recording film made of a magnetic material while projecting a light beam thereonto so as to cause a temperature rise at the irradiated area. Furthermore, the information is reproduced by projecting a light beam onto the recording film and detecting the rotation direction of the polarization plane of the reflected light.
The magneto-optical recording is roughly classified into two methods: i) the light modulation method wherein recording is executed by modulating the intensity of a light beam in accordance with information while maintaining an external magnetic field in a constant intensity and in a fixed direction; ii) the magnetic field modulation method wherein recording is executed by inverting the direction of the external magnetic field in accordance with information while maintaining a light beam in a constant intensity.
In the light modulation method, wherein the direction of the external magnetic field is fixed, an erasing operation is necessary for aligning the magnetization in the reversed direction to the direction of the external magnetic field, prior to the recording operation.
On the other hand, in the magnetic field modulation method, since new information can be recorded while erasing previously recorded information, no erasing operation is necessary prior to recording operation; thus, so-called overwriting is possible. Since recording time is shortened by the use of this method, the study of magneto-optical recording apparatuses using the magnetic field modulation recording has been actively carried out in recent years.
In the magneto-optical recording apparatus using the magnetic field modulation recording, the external magnetic field is normally generated by an electromagnet that is composed of a coil wound around a cylindrical ferrite core. The electromagnet can easily invert the direction of a generated magnetic field by switching the polarity of the current to be supplied to the coil.
Here, in a practical operation, the direction of the external magnetic field is inverted not instantaneously, but in a finite time. That is, when the external magnetic field is inverted, the intensity of the magnetic field diminishes as time elapses, reaching zero at a certain point of time, and then increasing. Therefore, centered around the point of time at which the intensity of the external magnetic field becomes zero, there exists a period during which the intensity of the external magnetic field becomes weaker than that required for recording.
For this reason, the chief disadvantage of this conventional arrangement is that recording bits having nonuniform shapes are formed in a recording area on the recording film whereonto this weaker external magnetic field has been applied; this causes deterioration of the signal quality in reproduction.
In order to solve the above problem, there has been proposed a method for lowering the intensity of a light beam when the direction of the external magnetic field is switched (U.S. patent application Ser. No. 07/738,923). With this method, since the intensity of the light beam is lowered upon inversion of the recording magnetic field that causes a weaker magnetic field, the disadvantage of having recordings made by the weaker magnetic field can be eliminated.
However, in the case of overwriting operation by the use of the above method, that is, in the case of making new recordings over previously recorded information, the inversion of the magnetization does not occur at an area in a previously recorded bit that has not been subjected to a light beam having a sufficient light intensity for recording. Consequently, a problem arises in how much of the previously recorded information remains after an overwriting operation ( this rate, hereinafter, referred to as the erasing rate).
Referring to FIG. 9, the following description will discuss the problem of the erasing rate in detail.
FIG. 9 illustrates recording conditions of a shortest recording bit 51 and a longest recording bit 52 which are determined by the modulation method adopted. When an external magnetic field, indicated by FIG. 9 (b), is applied while keeping the intensity of a light beam at the same high-level "H" as indicated by FIG. 9 (a), the resulting longest recording bit 52 widens toward its end portion as indicated by FIG. 9(c) because the area being subjected to a temperature rise, due to the irradiation by the light beam, widens toward its end portion.
For this reason, as illustrated in FIG. 10, when a signal corresponding to the shortest recording bit is overwritten on the longest recording bit 52, only the information corresponding a portion of the longest recording bit, located within a width W, that is, the width of the shortest recording bit, is erased and previous recordings are left at areas outside the width W (indicated by hatching for convenience).
Moreover, in addition to the problem of the remaining of previously recorded information, another problem is presented in that, since the width of a recording bit increases with the increase of the lengths of the recording bit, crosstalk is increased in reproducing signals of the adjoining tracks; this also causes deterioration of the signal quality in reproduction.